Study of Rectangular Underexpanded Microjets

The underexpanded microjet emerging from a rectangular convergent nozzle with a high aspect ratio at the nozzle exit is investigated numerically using the Reynolds-averaged Navier-Stokes (RANS) simulation with the Menter’s shear stress transport (SST) k-ω turbulence model. The simulation is performed at the nozzle pressure ratio of 5.0 to produce a strong shock and it is validated by a comparison with a rainbow schlieren picture of the microjet. The three-dimensional structure of the shock-containing rectangular microjet is demonstrated using the isopycnic surface and bright-field schlieren representations.

Flow Dynamics of a Spiral-groove Dry-gas Seal

The dry-gas seal has been widely used in different industries. With increased spin speed of the rotator shaft, turbulence occurs in the gas film between the stator and rotor seal faces. For the micro-scale flow in the gas film and grooves, turbulence can change the pressure distribution of the gas film. Hence, the seal performance is influenced. However, turbulence effects and methods for their evaluation are not considered in the existing industrial designs of dry-gas seal. The present paper numerically obtains the turbulent flow fields of a spiral-groove dry-gas seal to analyze turbulence effects on seal performance. The direct numerical simulation （DNS） and Reynolds-averaged Navier-Stokes （RANS） methods are utilized to predict the velocity field properties in the grooves and gas film. The key performance parameter, open force, is obtained by integrating the pressure distribution, and the obtained result is in good agreement with the experimental data of other researchers. Very large velocity gradients are found in the sealing gas film because of the geometrical effects of the grooves. Considering turbulence effects, the calculation results show that both the gas film pressure and open force decrease. The RANS method underestimates the performance, compared with the DNS. The solution of the conventional Reynolds lubrication equation without turbulence effects suffers from significant calculation errors and a small application scope. The present study helps elucidate the physical mechanism of the hydrodynamic effects of grooves for improving and optimizing the industrial design or seal face pattern of a dry-gas seal.

Study of Tunnel Thruster Performance and Flow by Quasi-Steady Reynolds-Averaged Navier-Stokes Simulation

A numerical approach based on the solution of the Reynolds-averaged Navier-Stokes(RANS) equations using the shear-stress transport(SST) turbulence model has been employed to investigate the hydrodynamic performance and flow of tunnel thrusters.The flow passages between adjacent blades are discretized with prismatic cells so that the boundary layer flow is resolved down to the viscous sub-layer.The hydrodynamic performances predicted by the quasi-steady approach agree well with the experimental data for three impellers covering a range of blade area and pitch.Through analysis of the flow field,the reason why the hub of impeller also contributes to thrust which can amount to 40%—60% of the impeller thrust,and the mechanism of the impeller inducing an axial force on the hull are elucidated.

Numerical simulations of single and multi-staged injection of H_(2) in a supersonic scramjet combustor

Computational fuid dynamics(CFD)simulations of a single staged injection of H_(2) through a central wedge shaped strut and a multi staged injection through wall injectors are carried out by using Ansys CFX-12 code.Unstructured terahedral grids for narow channel and quarter geometries of the combustor are generated by using ICEM CFD.Steady three dimensional(3D)Reynods averaged Navier-stokes(RANS)simulations are carried out in the case of no H_(2) injection and compared with the simulations of single staged pilot and/or main H2 injections and multistage injection.Shear stuess transport(SST)based on k-ω turbulent model is adopted.Flow field visualization(omplex shock waves interactions)and static pressure distribution along the wall of the combustor are pradicted and compared with the experimental schlieren images and measured wall static pressures for validation.A good agreement is found between the CFD predicted results and the measured data.The narow and quarter geometries of the combustor give similar results with very small differences.Multi-staged injections of H_(2) enhance the turbulent H_(2)/air mixing by fomming vortices and additional shock waves(bow shocks).